In order to ‘synthesize’ a particular optical pulse form one needs to be able to reliably define the amplitude and phase profile of an optical field. The general approach is to generate pulses with a well-defined pulse form and to then pass the pulse through some pulse shaping element with an appropriately designed transfer function to re-phase and re-shape the incident spectrum so as to obtain the desired output optical field. The pulse-shaping element can have a pure linear response such as a filter with a suitably complex response, or might additionally include a non-linear element, e.g. an optical fibre or an aperiodic quasi-phase matched structure, to allow the controlled generation of frequency components outside the frequency spectrum of the input pulse-form.
The most commonly used technique is a simple linear filtering technique in which the frequency components of a short pulse are spatially dispersed using bulk gratings, and then filtered by means of amplitude and phase-masks positioned within a Fourier optical 4 f set-up. Microlithographically fabricated spatial-masks, segmented liquid crystal modulators, or acousto-optic modulators have been used as spatial filters, the latter two approaches allowing for programmability and dynamic reconfigurability of the pulse shaping response. Whilst impressive results are possible with this approach, the hardware itself is somewhat bulky, lossy and expensive and does not lend itself to ready integration with waveguide devices. These issues have prompted the search for other technical approaches to the problem such as the use of arrayed waveguide gratings, and arrays of fibre delay lines.
Single channel data rates approaching the Tbit/s level have now been reported for optical time division multiplexing (OTDM) systems. These single channel data rates place increased demands and tolerances on the techniques used to multiplex and demultiplex the optical data bits. Consider for example the case of optical demultiplexing. As OTDM data rates increase, and the pulses used get correspondingly shorter, the synchronization requirements placed on the locally generated pulses used to control the switch operation can become a limiting practical issue. The key to reducing time jitter tolerance in such devices is to establish a rectangular temporal switching window. This reduces the absolute accuracy for temporal bit alignment and provides optimal resilience to timing jitter-induced errors. Fibre based non-linear optical loop mirror (NOLM) demultiplexing schemes that provide both good, ultra-fast performance and tolerance to timing-jitter of either or both of the control and data signals have been demonstrated previously. These schemes use the difference in group velocity and the resultant ‘walk-off’ between the control and data signals within the non-linear optical device to define the rectangular switching window. This consequently requires tight specification and control of both the data and signal wavelengths, and the dispersion characteristics of the fibre. Whilst this approach is applicable to fibre based switches, it is complex and cannot be applied to switches based on highly non-linear semiconductors and within which there are no appreciable dispersive propagation effects over the length scales of relevance. Simple, robust techniques that can help reduce time jitter tolerances and that are applicable to a variety of switching mechanisms are thus of great interest.
It is an aim of the present invention to obviate or reduce the above mentioned problems.